82 



THE DEVELOPMENT OF THE CHICK 



of the notochord remains attached to the entoderm for a consid- 

 erable period after the formation of the head-fold. A longitudinal 

 section shows the head-process as an appendage to the anterior 

 end of the primitive streak, or the primitive knot (Fig. 42). 



m.n. 



FIG. 43. Diagrams to illustrate the theory of concrescence as applied to 

 the primitive streak of the bird. The central area bounded by the broken 

 line represents the pellucid area; external to this is the area opaca, showing 

 as concentric zones the germ-wall (G. W.), the zone of junction (Z. J.), 

 and the margin of overgrowth (M. O.). m. n., Marginal notch. For de- 

 scription see text. 



The most obvious interpretation of the head-process is as 

 an outgrowth from the primitive knot. But another, and more 

 probable interpretation in view of all the facts, is that the head- 

 process is a later stage of the anterior end of the primitive streak; 



